(1) Field of the Invention
This invention concerns methods for manufacturing propylene glycol through the hydrogenolysis of glycerol.
(2) Description of the Art
The production of biodiesel utilizes vegetable oils, fats and waste restaurant greases while reducing the U.S. dependence on foreign crude oil. Biodiesel is a renewable, alternative fuel that reduces particulate matter and hydrocarbon emissions. However, for every 9 kilograms of biodiesel produced, about 1 kilogram of a crude glycerol by-product is formed.
A problem that results from refining this crude glycerol into refined glycerol is that the glycerol market cannot absorb it. With plentiful glycerol available, its price and U.S. exports have declined. As a result, much of the crude glycerol by-product of biodiesel production is currently disposed of or is sold at a very minimal price.
This problem may continue to worsen because the U.S. production of biodiesel is expected to continue to grow with a target of 400 million gallons of production by the year 2012. At this production capacity, 3.5 million gallons of crude glycerol will be produced every year. This crude glycerol can be purified by several steps including vacuum distillation to produce USP grade glycerol. However, refining the crude glycerol is complex and expensive.
A problem with crude glycerol from a biodiesel plant is that it requires costly upgrading to achieve a technical grade or USP grade glycerol. Typically, biodiesel producers will acidulate the crude glycerol to remove fatty acids in order to facilitate methanol recovery and recycle. Additional steps must be taken to convert the crude glycerol into a high purity glycerol such as USP glycerol. These additional process steps—which increase the cost of producing USP glycerol—may include ion exchange and/or fractionation. There would be significant operating and capital cost incentives if biodiesel derived glycerol could be sold at a profit as a lower grade product or feedstock such as acidulated glycerol rather than requiring purification.
Glycerol can be converted into propylene glycol by well known methods such as hydrogenolysis. Propylene glycol is a major commodity chemical with a growing market and with an annual production of over 1 billion pounds in the U.S. alone. Some typical uses of propylene glycol are in unsaturated polyester resins, functional fluids (antifreeze, de-icing, and heat transfer), pharmaceuticals, foods, cosmetics, liquid detergents, tobacco humectants, flavors and fragrances, personal care, paints and animal feed.
Today, biodiesel production plants are in need of methods to realize increased income from this bio-based crude glycerol byproduct. If bio-based crude glycerol could be efficiently converted to propylene glycol by hydrogenolysis, then the technology could be used in biodiesel production plants to increase profitability. There is a need, therefore, for improvements to existing glycerol to propylene glycol hydrogenolysis processes so that the processes can operate efficiently and economically using glycerol feedstocks and in particular crude glycerol feedstocks that have been minimally acidulated.